Method of making a linear film potentiometer having a circular configuration

ABSTRACT

Extreme linearity of a film potentiometer is instantaneously obtained by a computerized sectioning method. A circular film potentiometer track is sectioned off for the purposes of trimming the track, the number of sections being determined by the extent of linearity desired. A computerized contact point board programmed to produce a particular linearity for the circular track is then brought into contact with the track. On contact, current begins to flow through the resistance track producing a particular voltage at each sectionalized contact point area thereon. The voltage produced at each contact area, with reference to ground, is instantaneously compared to an electronically generated desired voltage. Every time the voltage across a particular section varies below the standard or desired comparison voltage, a laser beam cuts radially into the potentiometer track until the desired voltage is achieved. The laser beam moves along through each section of the track and the procedure advantageously provides reproducible results therefor, instantaneously.

United States Patent [191 Hukee et al.

1:1 3,821,845 45] July 2, 1974 METHOD OF MAKING A LINEAR FILM POTENTIOMETER HAVING A CIRCULAR CONFIGURATION [75] Inventors: Vernon V. Hukee, Nashua, NH;

William J. Travis, Ronse, Belgium [73] Assignee: Sprague Electric Company, North Adams, Mass.

[22] Filed: June 29, 1972 [2]] Appl. No.: 267,277

Related US. Application Data [63] Continuation-impart of Ser. No. [27,106, March 23 I971, abandoned.

[56] References Cited UNITED STATES PATENTS l0/l965 Martin 338/195 X 3/1972 Cardell 338/195 X Primary Examiner-Roy Lake Assistant Examiner -lames W. Davie Attorney, Agent, or Firm-Connolly and Hutz 5 ABSTRACT Extreme linearity of a film potentiometer is instantaneously obtained by a computerized sectioning method. A circular film potentiometer track is sectioned off for the purposes of trimming the track, the number of sections being determined by the extent of linearity desired. A computerized contact point board programmed to produce a particular linearity for the circular track is then brought into contact with the track. On contact, current begins to flow through the resistance track producing a particular voltage at each sectionalized contact point area thereon. The voltage produced at each contact area, with reference to ground, is instantaneously compared to an electronically generated desired voltage. Every time the voltage across a particular section varies below the standard or desired comparison voltage, a laser beam cuts radi-,

ally into the potentiometer track until the desired voltage is achieved. The laser beam moves along through each section of the track and the procedure advantageously provides reproducible results therefor, instantaneously.

l Claim, 3 Drawing Figures Motion Canimi Yblllove LmserAlwgbwtdegfIra ak Zms'er Source Gut To "5, 5 5 QffzmdOlb A Aataal/ Volttfye Desired Controb gm'oss Volhge For l Power onhzct Compar- Sed wy' lll'lle Points aior Genezaie 'Mecllmdwm t PATEN'WDJUL" 2 mm Lcas'erAlozgbzs'ide of film ah A laser Source Gui To Z 7 QffmuiOlb Ac'tuaL Voltage Desired ControL Across Voltage For )0 Power Contact Compar- Elecll'om'mily Sectiozzirg III'L'6 Points odor Generaie Mechanism METHOD OF MAKING A LINEAR FILM POTENTIOMETER HAVING A CIRCULAR CONFIGURATION CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of US. Pat. application Ser. No. 127,106 filed Mar. 23, 1971 now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a method of making a linear film potentiometers and more particularly to a method of making circular film potentiometers so as to attain extreme linearity.

A potentiometer consists of a resistance element having a pair of terminals at opposite ends thereof and a moving contact such as a multi-contact metal brush which may be adjusted in position between the end terminals. The present invention is concerned with a film potentiometer of circular configuration that is extremely linear and employs a metal contact brush carried by a rotary shaft.

Prior art potentiometers of this nature have had some difficulty in obtaining extreme linearity 1 percent) when resistance inks are used, such as metal film or cermet ink systems. And while some attempts at achieving linearity have been successful with certain resistance films, no method of achieving linearity has been found that is satisfactory for all resistance inks used in the manufacture of film potentiometers of circular configuration. Further, no reproducible and rapid method of producing extremely linear film potentiometers has yet been found. Heretofore, resistance films have been trimmed to a pre-specified value, but circularly configurated variable resistors have been difficult to adjust along the entire resistance track so as to achieve linearity thereon, and no satisfactory method has yet been found to accomplish this.

Accordingly, it is an object of the present invention to provide a potentiometer of circular configuration that is capable of operating at extreme linearity, i.e., 1

percent or less.

It is a further object of this invention to provide a method of instantaneously adjusting the, resistance 'of film potentiometers for any, and all, ink systems in such a manner as to achieve extreme linearity therefor.

SUMMARY OF THE INVENTION A circular film resistance track is theoretically divided up into a particular number of sections and is brought into contact with a computerized contact point board. The number of contact points on the board is equal to the number of sectionalized divisions on the track, and the board is programmed to have a particular desired comparison voltage across each contact area. A current begins to flow through the resistance track when it is brought into contact with the contact board, and a voltage measurement, with reference to ground, is made through each sectionalized division on the track. By having a computerized controlling means monitor and compare this voltage value, a cutting source, such as a laser beam, moves radially into the resistance track at each section every time the measured voltage value falls below the desired comparison voltage. The laser is electrically connected to the comput- BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a film potentiometer of the type that can utilize this functional technique of adjusting the linearity of the unit;

FIG. 2 is a perspective view of the contact point board used in this invention; and

FIG. 3 is a schematic illustration-of the mechanisms involved in this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1, there is illustrated a resistive film variable resistor, or potentiometer comprising an insulating substrate 12, an elongated, circular resistance track of a resistive film 11 having terminals 18 and 18a on opposite ends thereof and having a spaced gap between them. The film l 1 may be of any suitable resistance ink, such as a metal film or a cermet system. The film 11 may be formed on the insulating substrate 12 by any convenient or suitable method, such as by screening or by vacuum deposition. Screening is preferred because it is a more economical operation for applying the film to the substrate. However, care should be exercised when screening is used because this technique can present problems that must be overcome to achieve extreme linearity, that is, problems such as controlling the thickness of the film because of the mesh used invthe process. A contact brush 13 having, for example, a multi-contact metal brush is supported by a radially extending holder 17, extending from'a hub 14 pivoted on a shaft 15, and is capable of scanning the entire potentiometer track from terminal 18 to terminal 18a. The brush 13 is in direct electrical contact with the track from one terminal to the other. v

A potentiometer can be made extremely linear by bringing the resistance track to be adjusted into electrical contact with a contact point boardthat is electrically connected to a computerized control unit that can signal a cutting source to trim the track until extreme linearity is achieved all along the track. FIG. 2 shows a contact point board 20 that is made of electrically in sulative material 21, and has a generally circular configuration that is substantially the same size and shape as the potentiometer track to be adjusted. The contact point board 20 should be slightly larger diameter than the. track so as to pennit a laser beam, or other cutting source, to reach the resistance track when the contact board 20 is placed on top of same. A plurality of pairs of spaced apart, commerically available, spring-loaded metal contact points 22, or the like, are attached in parallel onto the board 21 in such a manner that they tilt slightly toward the center of the board. This is done to permit the inner portion of the track to be exposed when the board contacts the track prior to adjustment. The number of contacting areas on theboard is dependent on the extent of linearity desired, and will increase as the linearity approaches zero. Each pair of contact points 22 can be electrically contacted from above and r a voltage across these points, with reference to ground, can be measured and compared with a desired voltage at this particular point. It is in this manner that the resistance canbe measured and adjusted so as to attain linearity.

This process can better be described and understood by reference to FIG. 3, wherein a film resistance track 31 being of a circular configuration and having a resistance value somewhat lower than the desired value, is positioned on a supporting means, and is raised to come into electrical and physical contact with 'a contact point board 32. The board 32 has n number of contacting areas thereon. Each contact area is represented herein by two metal spring-loaded contact points that come into electrical and physical contact with a particular portion of the resistance track 31. These contacts are shown in the drawing at 33, 33a, 33b, 33c, 33d, 33e, 33f, 33g, 33h and 33i. The individual pairs of contacting points are connected in parallel and are in electrical connection with a computerized controlling means 40 using wires 34. A constant current source 35 then begins to flow through the resistance track, and the voltage across the contact points at 33 is registered in a differential amplifier comparator within the computerized controlling means 40. This measurement is instantly compared therein with an electronically generated desired voltage for that particular section of the track 31. A variation of the actual voltage from the desired voltage triggers a laser beam cutting source 51 to move radially into the resistance track 31 until the desired voltage is reached. The computerized controlling means 40 and contact point board 32 are programmed to cause both the voltage measurement and cutting operation to take place at each section, and through n sections, in a matter of seconds.

The computerized controlling means 40 includes provision for measuring the voltage at each of the contact areas on the board; provision for producing a desired comparison voltage; a comparator, such as a differential amplifier, for comparing the actual and desired voltages; and controls for moving a cutting source, such as a laser beam, both into or out of the resistance track, as well as moving it along the track. All switching from section to section and the like is done by a diode matrix in the controlling means. The computerized controlling means 40 is programmedto move in a particular manner through a particular predetermined number of sections, and to produce a particular linearity. The contact point board 32 is made to have a number of contacts equal to the number of .sections on the track and, when used in conjunction with a particular computer program, can be made to reproduce any number of potentiometers of the same linearity.-

In this embodiment, a laser cutting source is used wherein a laser beam 51 is bent off a mirror or prism 52 onto the resistance track-31. However, it should be pointed out that other cutting sources could be used with similar, although somewhat less desirable, results. A laser beam is preferable herein, however, because of its capability to make fine, accurate and precision cuts on the resistance track without interfering at all with the contact board. Also, the laser allows a much faster trimming operation than other cutting sources available. I

The constant current source 35 flows onto the potentiometer to be trimmed and produces a voltage at each contact area, with reference to ground, that will be V I X R, where V is the voltage, I is the constant current, and R is the resistance at that point on the track, relative to the grounded terminal. This measured voltage is immediately fed back to the computerized controlling means and into a differential amplifier comparator to be compared with an internally electronically generated desired voltage. Voltages measured at each of the contact point areas that are below the voltage desired at each area will signal the laser cutting source to move radially into the resistance track so as to cause the potentiometer to be trimmed to achieve the same voltage value as the desired .value. This procedure is continued at each contact area and in a matter of seconds an extreme linearity is achieved for the entire track that can be readily reproduced.

In this manner, contact point boards and accompanying computer programs can be stored, and at a subsequent date be used to advantageously reproduce large' quantities of potentiometers having a particular linearity.-,The big advantages of this method of attaining linear film potentiometers is that the trimming operation is instantaneously carried out along the entire resistance track, and that it reproducibly provides potentiometers having a degree of linearity that has not been attainable heretofore.

The above-described specific embodiment of the invention has been set forth for the purposes of illustration. It will be apparent to those skilled in the art that various modifications may be made in the process of adjusting the linearity of circular film resistance tracks without departing from the principals of this invention .as pointed out and disclosed herein. For that reason, it

is not intended that the invention should be limited other than bythe scope of the appended claim.

We claim:

1. A method of making a linear film potentiometer having a circular'configuration comprising:

I a. depostinga circularly configuratedfilm resistance track onto an insulating substrate, said resistance track being of a resistance value less than the particular desired resistance value and having terminals on opposite ends thereof and having a spaced gap between said terminals; and

b. laser beam cutting into said track beginning in a region adjacent to one of said terminals, progressing at a constant radius in said circular track toward the other of said terminals, said laser beam cutting being stopped periodically at predetermined locations, moving said cutting during each stopped interval as required to cut in a radial direction such that the width of said track is altered when said cutting subsequently again progresses in each instance from one of said predetermined locations, said width being determined by the direction and amount of said radial movement which in turn is controlled according to apredetermined linear formula having an independent variable that is the resistance difference-between a predetermined resistance and the actual resistance between said first terminal and the location in said track at which said cutting last stopped, said actual resistance being measured during each of said stopped intervals by a measuring means comprising a contact point contacting said track at each of said predetermined locations. 

1. A method of making a linear film potentiometer having a circular configuration comprising: a. deposting a circularly configurated film resistance track onto an insulating substrate, said resistance track being of a resistance value less than the particular desired resistance value and having terminals on opposite ends thereof and having a spaced gap between said terminals; and b. laser beam cutting into said track beginning in a region adjacent to one of said terminals, progressing at a constant radius in said circular track toward the other of said terminals, said laser beam cutting being stopped periodically at predetermined locations, moving said cutting during each stopped interval as required to cut in a radial direction such that the width of said track is altered when said cutting subsequently again progresses in each instance from one of said predetermined locations, said width being determined by the direction and amount of said radial movement which in turn is controlled according to a predetermined linear formula having an independent variable that is the resistance difference between a predetermined resistance and the actual resistance between said first terminal and the location in said track at which said cutting last stopped, said actual resistance being measured during each of said stopped intervals by a measuring means comprising a contact point contacting said track at each of said predetermined locations. 